This proposal is a continuation of our studies concerned with the isolation of components of the liver microsomal fatty acid chain elongation system. Our recent discovery of a microsomal short chain fatty acid reduction system which catalyzes reactions that are identical to those found in the long chain fatty acid elongation system, has made the separation of components no simple task, and has made us aware of the ever increasing complexity of the liver endoplasmic reticulum. Our primary focus will be the separation of these two systems, i.e., the separation of the two Beta-keto acyl CoA reductases (the short-chain Beta-keto reductase which we have called acetoacetyl CoA reductase versus the long chain Beta-keto reductase which utilizes Beta-ketopalmitoyl CoA or Beta-keto stearoyl CoA as substrates), separation of the two dehydratases and the two trans-2, 3-enoyl CoA reductases. Purification of the individual component enzymes will be our continued goal, followed by reconstitution of the short-chain and long-chain systems. A role for cytochrome b5, its flavoprotein reductase and NADPH cytochrome P-450 reductase in the short chain reductase system will be assessed; the role of phospholipid and other lipids will also be investigated. Another major goal of the proposal is the elucidation of regulation or modulation of both the microsomal fatty acid chain elongation system and the short chain acyl CoA reduction system. To this end, we will determine the role of dietary factors, diabetes, insulin, and glucagon in the control of these two systems. The diabetic animal may be especially important in determining the physiological importance of our newly discovered short-chain acyl CoA reduction system, since an important substrate of this enzyme system is acetoacetyl CoA, a ketone body precursor. Finally, do pharmacologic agents, like clofibrate cholestyramine which induce hypolipidemia exert a significant effect on these two microsomal systems? Our proposal should contribute significantly to our understanding of lipid metabolism.